Phosphodiesterase (PDE) is an enzyme that hydrolyzes cAMP and cGMP that function as intracellular second messengers into 5′-AMP and 5′-GMP, respectively. PDE gene is constituted with 21 genes, and currently classified into 11 kinds of families based on the molecular structure of the enzymes. Furthermore, each PDE is classified into the following 3 kinds: 1) cAMP-PDEs (PDE4, PDE7, PDE8), 2) cGMP-PDE (PDE5, PDE6, PDE9), and 3) dual-substrate PDEs (PDE1, PDE2, PDE3, PDE10, PDE11), based on the substrate specificity.
cAMP and cGMP are involved in the control of various physiological functions such as control of ion channel, muscle relaxation, learning and memory function, differentiation, apoptosis, lipogenesis, glycogenolysis and gluconeogenesis. Particularly, they are known to play an important role in the differentiation and survival, as well as control of neurotransmission of the nerve cell (non-patent document 1). Phosphorylation of various molecules that control physiological functions such as transcription factors, ion channel and receptor, which is caused by protein kinase A (PKA) and protein kinase G (PKG), contributes to such control by cAMP and cGMP, and the amounts of cAMP and cGMP in the cell are under spatiotemporal regulation via generation by adenylate cyclase and guanylate cyclase in response to extracellular stimulations and degradation thereof by PDE (non-patent document 2). Since PDE is a sole enzyme that decomposes cAMP and cGMP in vivo, PDE is considered to play an important role in the regulation of cyclic nucleotide signaling.
PDE10A is a molecule cloned by 3 independent groups and reported in 1999 (non-patent documents 3, 4, 5). Expression analysis thereof has clarified that PDE10A shows high expression only in the brain and testis, and has a localized expression pattern in the PDE family (non-patent documents 6, 7). In the brain, both PDE10A mRNA and PDE10A protein show high expression in medium spiny nerve cells of the striatum (medium spiny neurons, MSNs) (non-patent documents 8, 9). MSNs are classified as two major kinds of pathways. One of them is called a direct pathway or nigrostriatal pathway, and mainly expresses dopamine D1 receptors. The other pathway, indirect pathway, is called a striatum-globus pallidus pathway, and mainly expresses dopamine D2 receptors. The direct pathway is involved in the functions of motion execution and reward learning and, on the other hand, the indirect pathway is involved in the suppression of motility. The activity of the output nucleus of the basal nucleus is regulated by the balance of antagonistic inputs from these two kinds of pathways. Since PDE10A is expressed in MSNs of both pathways, the both pathways are considered to be activated by inhibition of PDE10A. Since the action of existing antipsychotic agents having a D2 receptor shutting off action is mainly mediated by the activation of indirect pathway, a PDE10A inhibitor is expected to show an anti-mental disease action like existing drugs.
The excess D2 receptor shutting off action in the brain by existing drugs causes side effects such as hyperprolactinemia and extrapyramidal syndrome. However, since PDE10A shows striatum pathway specific expression and shows a lower expression level in the pituitary gland mainly involved in the prolactin release, PDE10A inhibitor is considered to have no prolactin concentration increasing action in plasma. Moreover, since PDE10A is also expressed in the direct pathway MSNs and activated by a PDE10A inhibitor, it is considered to have superior characteristics than existing antipsychotic agents that activate only indirect pathways. That is, since the direct pathway is involved in the motion execution, it is considered to antagonistically act against extrapyramidal syndrome caused by excessive activation of indirect pathway. Furthermore, this pathway is expected to show actions to enhance the output from the striatum-thalamus circuit and promote cognitive functions of reward learning and problem solving. Since existing antipsychotic agents show a shutting off action on many receptors, they pose problems of side effects such as body weight increase and abnormal metabolism. PDE10A inhibitor is also considered to be superior to the existing drugs in the side effects, since it directly activates second messenger signaling without blocking dopamine and/or other neurotransmitter receptors. In view of the specific expression and its function in the brain nerve system, PDE10A is considered to be useful as a drug discovery target in neurological diseases, in particular, psychotic disorders such as schizophrenia.
Patent document 1 discloses, as a PDE10A inhibitor, a compound of the following formula:
and the following compounds:

Patent document 2 discloses, as a PDE10A inhibitor, a compound of the following formula:
and the following compounds:

Patent document 3 discloses, as a PDE10A inhibitor, a compound of the following formula:
and the following compounds:

Patent document 4 discloses, as a PDE10A inhibitor, a compound of the following formula:
wherein Z is

Patent document 5 discloses the following compound as a PDE10A inhibitor.

Patent document 6 discloses, as a PDE10A inhibitor, a compound of the following formula:
and the following compounds:

Patent document 7 discloses 5-(cyclopropylmethoxy)-3-(1-phenyl-1H-pyrazol-5-yl)-1-[3-(trifluoromethyl)phenyl]pyridazin-4(1H)-one.
In addition, patent documents 8 and 9 disclose the following compounds which are different from PDE10A inhibitors in the use thereof.
patent document 8: inhibitors of Lp-PLA2 enzyme    (1) N-(1-ethylpiperidin-4-yl)-2-{6-[(4-fluorobenzyl)sulfanyl]-3-methyl-4-oxopyridazin-1(4H)-yl}-N-{[4′-(trifluoromethyl)biphenyl-4-yl]methyl}acetamide,    (2) ethyl {6-[(4-fluorobenzyl)sulfanyl]-3-methyl-4-oxopyridazin-1(4H)-yl}acetate, and    (3) {6-[(4-fluorobenzyl)sulfanyl]-3-methyl-4-oxopyridazin-1(4H)-yl}acetic acid.
patent document 9: antibacterial agents of cepham compound    (1) benzyl 7-{[(2Z)-2-(2-amino-1,3-thiazol-4-yl)-3-methoxyprop-2-enoyl]amino}-3-{[(6-chloro-2-methyl-5-oxo-2,5-dihydropyridazin-3-yl)sulfanyl]methyl}-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate 5-oxide,    (2) benzyl 3-{[(6-chloro-2-methyl-5-oxo-2,5-dihydropyridazin-3-yl)sulfanyl]methyl}-7-({(2Z)-2-[2-(formylamino)-1,3-thiazol-4-yl]-3-methoxyprop-2-enoyl}amino)-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate,    (3) 7-{[(2Z)-2-(2-amino-1,3-thiazol-4-yl)-3-methoxyprop-2-enoyl]amino}-3-{[(6-chloro-4-hydroxy-2-methyl-5-oxo-2,5-dihydropyridazin-3-yl)sulfanyl]methyl}-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid, and    (4) 7-[2-(5-amino-1,2,4-thiadiazol-3-yl)-2-ethoxyiminoacetamido]-3-(1-methyl-3-chloro-4-oxo-1,4-dihydropyridazin-6-yl)thiomethyl-3-cepham-4-carboxylic acid.